1. Field of the Invention
The present invention relates generally to a method for reducing the effect of electrical cross-coupling in micro-electromechanical systems, and more particularly to a modulation method for signal crosstalk mitigation in electrostatically driven devices.
2. Description of the Related Art
A micro-electromechanical system (MEMS) may be used to sense the changes in rotation of a resonant element, among other things, and may be fabricated using a variety of different structures (e.g., gyroscopes) as the resonant element. FIG. 1 is a block diagram of a prior art MEMS 100, which typically uses a primary device 105 having an electrostatic capacitive drive that receives excitation drive signals and produces a primary vibratory motion as detected by a primary pickoff signal (e.g., an oscillatory signal). The characteristics (e.g., amplitude and phase) of the primary pickoff signal may be sensed and controlled using a primary pickoff sensing device 110, which outputs an excitation motion measurement, which is used to set the basic motion amplitude. The primary device 105 may be coupled to a secondary device 115 (e.g., a Coriolis vibratory rate sensor) for producing a secondary vibratory motion responsive to an external parameter (e.g., angular rate in the case of a gyro). Such motion may be sensed and controlled using a secondary pickoff sensing device 120. The secondary pickoff sensing device 120 may be used to measure the characteristics of the secondary pickoff signal and may provide an open loop output at, for example, 2,000 Hz.
The MEMS 100 may also include a nulling servo 125 whose input is coupled to the secondary pickoff sensing device 120 and whose output is coupled to the secondary device 115. The nulling servo 125 receives the secondary pickoff signal and generates an oscillatory feedback signal that is used to null the secondary pickoff signal as sensed by the secondary pickoff sensing device 120. Consequently, a feedback signal, which becomes the measurement of the desired characteristic (e.g., angular rate measurement for a gyroscope) is produced at the output of the nulling servo 125 and is fed into the secondary device 115. The nulling servo 125 may also produce a closed-loop output.
The primary and secondary devices 105, 115 may include high Q mechanical systems that are used to provide mechanical amplification of the primary vibratory signal or the secondary vibratory signal or both. The high Q mechanical systems have peak responses at the resonant frequency leading to oscillatory motion that is substantially sinusoidal. Therefore, in either the open loop or closed loop configuration, the primary and secondary pickoff signals may be demodulated to extract or remove the excitation frequency (i.e., amplitude and phase) of the motion and obtain a measure of the motion carried by the excitation drive signal.
One drawback of conventional MEMS is the problems associated with electrical cross-coupling. Electrical cross-coupling often occurs because the MEMS devices and structures are very small and produce stray capacitances that are significant compared to the actual variable capacitance used for the primary and secondary pickoff signals. Also, the primary and secondary pickoff signals are much smaller than the excitation drive signal. Hence, electrical cross-coupling of the excitation drive signals into the primary and secondary pickoff signals is very likely and generally unavoidable. Thus, it should be appreciated that there is a need for a method for reducing the effect of electrical cross-coupling in micro-electromechanical systems. The present invention fulfills this need as well as others.